Changes in hydrological systems associated with climate change may threaten to reliability of water resources management systems. Sophistication of reservoir operation is considered to be important to enhance existing water resources management systems. In this chapter, an adaptive operation method of a multipurpose reservoir considering real-time hydrological prediction is presented for more robust water resources management under climate change. A method to design and assess integrated reservoir operation for both the flood control and water utilization is developed with consideration of imperfect real-time hydrological predictions in order to enhance the capability of an existing multipurpose reservoir.